Portable communication devices, such as cellular telephones, personal digital assistants (PDAs) and other communication devices often include multiple radio receivers or transceivers. For example, a cellular telephone may include a cellular transceiver, a television receiver and an FM radio. A significant challenge of a radio receiver design is that the receiver must perform adequately in the presence of adjacent channel interference (ACI) or other nearby channel interferences, such as alternate channel interference. Adjacent channel interference refers to RF energy that is present in the channels that are adjacent to the desired receive channel to interfere with the desired receive signal.
The presence of strong adjacent channel interference or other nearby channel interferences can result in significant energy spilling over into the desired receive channel. This degrades the signal to interferer ratio and the signal to noise ratio of the receiver, and thus degrades the performance of the receiver. One possible solution is to filter the received signal using a channel filter having a narrow bandwidth. A narrow bandwidth channel filter provides significant reduction of adjacent channel interference, but also attenuates the desired receive signal.
In the case of an FM radio receiver, a narrow bandwidth channel filter attenuates the stereo FM signal, which causes harmonic distortion and degrades channel separation in the audio. Therefore, it is desirable to minimize adjacent and nearby channel interference while maximizing the strength of the desired signal.